Mutation induction in relation to organ specificity, 2) mutation induction during development, 3) systemic effect of somatic mutations of age-related degenerative diseases, and 4) tumor progression from pre- neoplastic to neoplastic growth in relation to genomic stability. A recoverable vector transgenic approach can be used to deal with some of these issues. Use of vectors that only detect a limited array of genetic alterations give increased sensitivity in kinetic studies, require fewer data points, and uncover specific mutagenicity that may go undetected with targets that respond to a broad spectrum. For example, the A:T base pair plays an extremely important role in induced mutagenicity in mammals, but the effect of mutagens on the A:T target is often masked by C pG mutagenicity in transgenic systems that detect a broad spectrum of genetic alterations. Initially, the well characterized am3 mutation of phiX174 is used as a transgene to evaluate substitutions at the A:T base pair. An inbred line of C57BL6/J mice was established that is homozygous for phiX174 in a tandem array and free of detrimental effect from the insertion. Reversions via one transition and two transversions are detected by selection among progeny phage recovered from the transgenic animals. The vector can be recovered from the genomic DNA, and both recovery and putative mutation frequencies are independent of any endogenous variation in the CpG methylation between host animals and tissues. The level of CpG methylation increases progressively with the number of generations removed from the founder animal. To maintain high recovery of the vector from animals from subsequent generations it was necessary to develop a bacterial strain with a high methylation tolerance level for recovery of PhiX174 from the mammalian DNA. The spontaneous reverse mutation frequencies of am3 in the vector isolated from in testis, liver and spleen seems to be close to the same namely approximately 1 per million of recovered vectors. After treatment with ethyl nitrosourea the mutation frequency varied greatly between animals. This variation may indicate a strong clonial nature of the mutations.